This invention relates to the production of thermoplastic tubing and, more particularly, relates to a method and apparatus for fusing a bell connector onto a corrugated thermoplastic pipe.
Corrugated thermoplastic pipe such as that produced according to Patent Cooperation Treaty (PCT) Application Ser. No. PCT/CA 92/00422 filed Sep. 25, 1992 can be produced in any length, the length being determined by shipping limitations. Once at the installation site, lengths of pipe must be connected together axially.
Corrugated pipe of the type shown in the above-mentioned PCT application, particularly pipe of large diameter such as up to one meter in diameter, are difficult to interconnect with a structurally integral joint because of the size and shape of the pipe. There is thus a need for a reliable bell and spigot joint on thermoplastic pipe, particularly large diameter corrugated pipe, for facile assembly in the field.
Canadian Patent No. 144,585 discloses a method and apparatus for forming pipe couplings in which a bell or sleeve of a pipe coupling is formed at the end of a standard thermoplastic polyvinylchloride pipe. A tool is employed which comprises a mandrel having a diameter equal to the outer diameter of the pipe to be joined, of generally cylindrical configuration, with a forwardly and inwardly tapered leading end. The smaller diameter leading end forms a guide for a heated pipe as it is forced onto the mandrel and is subsequentially spread to the diameter of the mandrel. Further, apparatus is described to form a corrugation on the bell to provide a recess for an adhesive or other sealing means. The method and apparatus of this patent are limited to smooth wall pipe or tubing made of polyvinylchoride which have stability when heated and are not well suited for pipe or tubing made of polyolefins such as polyethylene and polypropylene which have poor thermal stability when heated to the fusion temperature.
A method for forming pipe couplings is described in U.S. Pat. No. 3,998,478 wherein a low cost gasket is provided at the joint coupling. The gasket has a simple sleeve-like internally ribbed form and essentially is self-holding and sealing in an annular space as defined between a plain spigot or male pipe end and a receiving bell or female formation of another element joined therewith. This gasket obviates the need for adhesive or solvent bonding of the pipe coupling. It is assumed in this disclosure that a sleeve or bell may be already present on the pipe end or may be readily formed thereon.
U.S. Pat. No. 3,841,668 discloses another pipe coupling assembly which provides a means to accommodate thermal expansion of the pipe. A transparent coupling welded or glued at one end to a first length of pipe has a seal ring in an annular groove in the other end of the coupling so that a second length of pipe can be inserted therein. The transparent coupling has indicia thereon to enable positioning of the second length of pipe at a proper location in accordance with the ambient temperature. Such a coupling may not work effectively on corrugated pipe since the sealing ring will not readily slip past each corrugation.
The fuse welding of pipe and tubing made of polyolefins is difficult to effect consistently because of the thermal instability of the material at fusion temperature, i.e. the inability of the polyolefins to maintain their shape when heated, and there is a need therefore to provide a method and apparatus for fusing a bell connector onto a thermoplastic pipe, particularly externally corrugated pipe having a smooth liner, made of polyolefins such as polyethylene and polypropylene. The bell connectors often are fuse welded to the front face of a corrugation. However, it has been found that the pipe corrugation may collapse when heated and joined to a bell under pressure.